The present invention is generally directed to a voltage monitor for sensing a DC voltage and producing a warning that the level of the DC voltage has fallen below a predetermined level for a predetermined percentage of time. The present invention is more particularly directed to monitoring a DC voltage produced by a battery, periodically comparing the battery voltage with a threshold voltage and storing the result of the comparison. If three or more of the last sixteen comparisons indicate a low voltage, the device warns the user of the abnormal condition.
There are many applications where it may be important to sense a voltage level and provide warning of an out-of-range condition. Any DC voltage, whether generated by an AC-to-DC power supply or a depletable source such as a battery may be monitored and a warning provided if the DC voltage varies over time. One such application is a battery-powered smoke detector, where the device could activate a visual or audible warning of a low state-of-charge battery condition.
In another particular application, in the case of a battery used to start an automobile, such a warning may alert the user to an impending "no start" condition, where the battery charge is insufficient to start the automobile. Such a condition may result from a slow drop in the battery's state-of-charge. The reduced charge may be the result of an electrical problem with the car, such as a slipping belt or a failed regulator. Alternatively, the reduced state-of-charge may be the result of battery failure, such as a broken weld or active material shedding. In such an application, a device which senses the abnormal battery voltage can alert the user to the condition so that corrective measures may be taken.
In the preferred embodiment of the present invention, the battery or power source is discharged at a particular current for large portions of time. In the smoke detector application, the battery is powering the smoke detector circuity for the majority of its time, and occasionally being tested or recharged. Similarly, in the Automotive application, the battery is powering electronic hardware during non-use, resulting in a constant discharge current for the majority of the time, interrupted only occasionally by the starting and driving currents. During the constant discharge portion, a battery voltage will change to a value representative solely upon its state-of-charge.
For example, if the digital battery capacity warning device reads the voltage every 2 hours, statistically it will scan at times when the battery is at this constant discharge current and the battery voltage will be representative upon its state-of-charge. In accordance with the preferred embodiment, the warning device scan rate is selected so that two of the sixteen values are taken during the time when the battery may have been in use, as for example, 5% of the time, with the other fourteen values being taken during the time when the battery has not been in use, as for example, 95% of the time.
This device is suited to many other applications where the battery is held at rest or under a constant discharge for significant portions of time. The number of low values can be altered to reflect the percentage of time the battery is not at this constant discharge state. Similarly the frequency of the scans or number of scans can vary based upon how quickly the device is expected to respond to an out-of-limit condition.
Devices for sensing low battery voltage are generally known. U.S. Pat. No. 4,902,956 issued to Sloan on Feb. 20, 1990, describes a voltmeter and circuit breaker wired in series with selected, nonessential accessories of an automobile. The voltmeter monitors the battery voltage and interrupts the flow of current to the accessories when the battery voltage falls below a predetermined threshold. Such devices rely on a single below threshold voltage measurement by the voltmeter to cause the circuit breaker to trip and the accessories to be disconnected from the battery. Unfortunately, conditions unrelated to the battery charge, such as momentary transients in the voltage seen by the apparatus, may cause the circuit breaker to trip. Such voltage transients may be caused by inductive drops caused by suddenly changing current levels, as when an accessory is turned on or off. Alternatively, such momentary drops may occur during starting when the battery voltage may drop to as low as six volts. This sensitivity to voltage transients is an inherent and undesirable limitation on such devices. Moreover, these devices must set their threshold voltage abnormally low to prevent false indications and thereby indicate only when the battery is fully discharged. The present invention can preset the threshold voltage to indicate when the battery state-of-charge is below a predetermined value ranging from 0 to 100% with the knowledge of the battery type and constant discharge current.